In many materials and devices, the electric current is a linear function of the electric field. These materials and devices are conventionally termed ohmic and have an electrical resistance which is independent of the applied voltage. There are, however, many materials and devices in which the electrical current is a nonlinear function of the electric field. In these materials and devices, conventionally termed non-ohmic, the electrical resistance is a function of the applied voltage.
Non-ohmic devices are numerous and extensively used in modern technology and include such well known devices as p-n junction diodes, tunnel diodes and metal-semiconductor devices. One non-ohmic device that is widely used is a two-terminal semiconductor device that is commonly called a varistor (variable resistor). This device is useful in applications such as equalization of the direct current through a piece of electrical equipment for different applied voltages.
The current-voltage characteristics of varistors are conveniently represented by the equation I=KV.sup..alpha.. In this equation, K is a proportionality constant which depends upon both the resistivity and the dimensions of the device, and .alpha. is an index of a nonlinearity of the current-voltage relationship and is itself usually a function of the current. If .alpha. is equal to 1 and independent of the current, the device is ohmic.
Several materials have been exploited in varistors. For example, varistors using silicon carbide (SiC) based material has been extensively used, particularly in telephone systems, to provide line equalization. That is, these varistors equalize the direct current from a central office to individual telephone units, typically within the customer's premises, located at different circuit path lengths. Silicon carbide devices are also used in telephone central offices to protect equipment such as switching systems.
Varistor devices based on silicon carbide, however, have several drawbacks. For example, it is rather difficult to incorporate the dopants that are essential for the desired varistor characteristics into the SiC grains. Furthermore, clay from natural sources is typically used as a binder and filler material and graphite is used as a reducing agent during a processing sequence. These additives present problems with respect to quality control. Additionally, an appropriate moisture content must be maintained in the samples prior to firing.
Recently, varistors using metal oxides have been developed. For example, see Japanese Journal of Applied Physics 10 pp. 736-746 (1971). In particular, zinc oxide (ZnO) ceramics with selected metal oxide additives have been described. It is currently believed by many persons in the art that the non-ohmic properties are caused by segregation of the additive oxides at the grain boundaries of the zinc oxide ceramics.
Zinc oxide devices, however, typically operate between 50 and 200 volts to produce the desired current-voltage characteristics. The voltage drop across each grain boundary in ZnO has been reported to be 2 to 3 volts, see Journal of Applied Physics 46(3), pp. 1332-1341(1975). Thus, operation at less than 20 volts requires a structure which is less than 10 grains thick and mechanically weak.
For many purposes, low voltage operation with varistors having reasonable mechanical strength is desired.
Metal oxide compositions are also useful in other electrical devices, such as capacitors. Commercial ceramic capacitors are generally either disc, multilayer or barrier with the latter two types generally having the highest capacitance per unit volume. Multilayer ceramic capacitors have become very expensive in recent years because a significant quantity of precious metal, used as electrode material and placed between the ceramic layers, is required to make a multilayer device. As a result, barrier layer capacitors have become at least as competitive as multilayer capacitors for many applications.
The dielectric properties of barrier layer capacitors result from the insulating layers formed at the grain boundaries of semiconducting grains. These grains have a high conductivity and form a support for the grain boundary dielectric layers. Typical barrier layer capacitors require two firings. in the first, the ceramic is sintered at a high temperature in a reducing atmosphere to yield large grain sizes and high electrical conductivity. In the second, low melting oxides are deposited on the sintered ceramic and annealed in an oxidizing atmosphere at a lower temperature. The oxide coating melts and penetrates the grain boundaries. Upon cooling, the low melting oxides form a dielectric second phase at the grain boundaries. The capacitors have a high capacitance because the second phase has dimensions that are small with respect to the grain size.
An alternative method of forming the thin insulating layers uses solute segregation at the grain boundaries as previously described for varistors.